Recently, a lithium secondary battery is generally employed as an electric source for driving small electronic devices. The lithium secondary battery essentially comprises a positive electrode, a nonaqueous electrolytic solution, and a negative electrode. A lithium secondary battery utilizing a positive electrode of lithium compound oxide such as LiCoO2 and a negative electrode of carbonaceous material or lithium metal is favorably used. As the electrolytic solution for the lithium secondary battery, a carbonate such as ethylene carbonate (EC) or propylene carbonate (PC) is favorably used.
Nevertheless, it is desired to provide a secondary battery showing improved characteristics in the cycle performance and electric capacity.
A lithium secondary battery utilizing a positive electrode of LiCoO2, LiMn2O4 or LiNiO2 sometimes shows decrease of electric performances because a portion of the nonaqueous solvent in the nonaqueous electrolytic solution oxidatively decomposes in the course of charging and hence the produced decomposition product disturbs the desired electrochemical reaction. The decomposition is considered to be caused by electrochemical oxidation of the solvent on the interface between the positive electrode and the nonaqueous electrolytic solution.
On the other hand, a lithium secondary battery utilizing a negative electrode of carbonaceous material of high crystallization such as natural graphite or artificial graphite also shows decrease of electric performances because a solvent of the electrolytic solution reductively decomposes on the surface of the negative electrode in the course of charging. The reductive decomposition also occurs in the repeated charging-discharging procedures when EC (which is generally employed as the nonaqueous solvent of the electrolytic solution) is utilized as the nonaqueous solvent.
Japanese Patent Provisional Publication 10-74537 describes that the cycle performance and electric capacity are improved when a small amount of an aromatic compound such as benzene having a hydrocarbon substituent (e.g., cyclohexylbenzene).
Japanese Patent Provisional Publication 10-112335 describes that the cycle performance is improved when a small amount of a fluorine atom-containing aromatic compound such as fluorobenzene is added to a nonaqueous electrolytic solution of a lithium secondary battery.